Weathering and geochemical fluxes in the Canadian Cordillera: Evidence from major elements, rare earth elements, mercury, and carbon and sulphur isotopes in the Fraser, Skeena and Nass Rivers.

摘要

Water and suspended particulate samples from the Fraser, Skeena and Nass River systems of the western Canadian Cordillera were analysed for dissolved major element concentrations (HCO3- , SO42-, Cl-, Ca2+, Mg2+, K+, Na+); delta 13C of dissolved inorganic carbon (DIC); and delta34S of dissolved sulphate (SO42-); rare earth elements (REE)---dissolved and adsorbed to the suspended particulate matter; and mercury (Hg)---dissolved, and associated with suspended particulate (adsorbed, organic bound, mineral bound). This data is used to quantify chemical weathering rates and exchanges of CO2 between the atmosphere, hydrosphere, and lithosphere; to assess weathering and erosion processes in alpine versus valley regions of the watersheds; and to quantify the transfer of Hg from rocks to the surface environment. The main results of this work are: (1) Important weathering processes in the Canadian Cordillera are: (i) carbonate dissolution by carbonic acid > (ii) silicate dissolution by carbonic acid ≈ (iii) carbonate dissolution by sulphuric acid derived from the oxidation of sulphides (coupled sulphide-carbonate weathering); (2) REE patterns in the dissolved and adsorbed phases (labile REE) of the rivers vary widely across the Cordilleran watersheds, and predictably reflect basin lithology. Main stem samples reflect mixtures of the tributaries, suggesting labile REE patterns are conservative in the rivers. This supports using labile REE as a weathering and hydrographic tracer; (3) Kdads/disREE values in June (high water stage, average ≈ 1 x 10 3) are approximately 50 times lower than Kdads/disREE values in October (low water stage, average ≈ 50 x 10 3) indicating a change in composition and source of the suspended particulate matter from recently eroded rock flour (alpine source) in June, to older alluvial soils (valley source with a higher percentage of clay minerals) in October; (4) Annual Hg export to coastal waters by these rivers is 11 x 103 kg·yr-1, of which 6% (650 kg·yr-1) is in labile phases (dissolved adsorbed organic bound). Mass balance indicates that 2.5 to 100% of this flux can be attributed to chemical weathering. However, the strong correlation between chemical weathering rates and Hg fluxes suggests that weathering is the dominant control (near to 100%) on Hg release and transport; (5) As with the REE, seasonal variation in Hg concentrations and distribution exist. In June, labile Hg dominantly exists in the adsorbed phase, but dominantly in the organic bound phase in October, which indicates Hg has greater affinity to organic ligands than direct adsorption to inorganic particulate surfaces. (Abstract shortened by UMI.)